Dry Cooling for Consentrated Solar Power a Must in Desert Climates

Concentrating solar power (CSP) “farms” use a lot of water. This is doubly distressing for many environmentalists because they are frequently sited in desert or near-desert areas, which offer limited habitability for humans but – as a result – are excellent wildlife refuges.

CSP sites can use upwards of 500 million gallons of water a year for cooling, or about 800 gallons per megawatt hour, and there are currently 34 such projects slated for development in the California and Nevada deserts – one, in the Mojave Desert having already been successfully blocked by Dianne Feinstein (D-CA; a move which prompted California Governor Arnold Schwarzenegger to note: “If we can’t put solar in the desert, I don’t know where the hell we can put it.”

There are several types of CSP. One is the parabolic trough, which uses long sections of curved reflective material oriented toward the sun. These troughs don’t need to orient on the vertical axis, since some portion of the sun is always hitting some part of the trough. They do orient horizontally, or perpendicularly, from morning to evening to match the sun’s crossing the sky (as in the case of Andasol 1 in Spain).

The reflected sunlight then heats a medium (i.e., oil) inside a pipe running the length of the trough, which in turn is used to produce steam to drive a turbine generator. Some plants also use molted (melted) salt to store heat for nighttime or cloudy-day electricity generation.

A second type is a power tower, which uses heliostats (mirrors) to concentrate solar energy on a collector tower. This method can require less level ground, and generate higher temperatures than parabolic-trough solar collection.

A third type, called a dish design, uses the same mirror principal, as a collection of mirrors in a roughly circular shape, to focus solar energy on a central receiver located close to the center of the dish but extended by about 20 feet on an armature.

Fresnel reflectors adopt the parabolic design, using slightly curved or even flat mirrors, in rows, to focus solar energy on a receiver, which may or may not have its own intensifying reflector. Linear Fresnel reflectors offer an adaptation of this technology using long tower receivers.

Perhaps the most efficient design to date is the Stirling engine/solar dish, which has a reported conversion rate (from solar insolation to electricity) of 31.25 percent. However, none of the designs mentioned is especially good at conserving water used to remove heat produced in the steam cycle.

The alternative is air, or dry, cooling, which BrightSource is using at its Ivanpah installation in the Mojave. These indirect dry cooling systems (Heller systems) can reportedly reduce water usage by a whopping 97 percent. Unfortunately, air cooling reduces efficiencies, by up to 5 percent; not a huge loss, until one considers the cost/benefit ratio of CSP plants. The loss is greater in very hot climates like the Mojave, and less so in climates where the nights are cooler (New Mexico, for instance).

Efficiencies can also be improved, according to energy consulting firm WorleyParsons, by creating a larger collector field or device to provide greater steam cycle performance.

Unfortunately, many developers are skeptical of these alternative-to-water cooling systems, and even view hybrid (air-water) cooling systems askance, which has so far prevented their support among environmentalists and legislators seeking to conserve precious water resources for a thirsty constituency.